The invention relates to a method for determining the flux vector of a rotating field machine from the stator current and the stator voltage and the apparatus for implementing the method, as well as to an application of the method and apparatus.
Such a method is used with the apparatus described in German Offenlegungsschrift No. 30 26 202 for the field oriented operation of a rotating field machine fed by a (frequency) converter. For controlling the field orientation, the position of the flux vector is determined and the converter feeding the machine is controlled as a function of the position of the flux vector in such a manner that the component of the stator current parallel to the flux and the stator current component perpendicular thereto can be influenced independently. Through the control of the stator current component parallel to the flux (magnetizing current), a pre-set value for the magnitude of the flux can be adjusted, while the current component perpendicular to the flux (active current) then enters into the torque linearly and can be used directly for the decoupled control of the speed of rotation or the torque.
However, knowledge of the position of the flux vector is necessary for this field orientation. In this connection it is advantageous to measure the flux, not directly via Hall probes, but to calculate it from electrical variables by means of a computer model circuit. The simplest possibility to accomplish this end is a so-called "voltage model" which determines the induced EMF from the input voltages of the motor by subtracting the ohmic stator voltage drop and the inductive leakage voltages. The flux is then obtained as the integral of the EMF.
For describing the machine currents, machine voltages, the EMF and the flux, plane vectors can be used each with two defining variables, for instance, their Cartesian or polar components with respect to a stationary (i.e., stator oriented coordinate system "fixed in space") or one corotating with the rotor shaft ("rotor oriented") or one rotating with the field axis ("field oriented"). For the mentioned "voltage model", consideration in the stator-oriented Cartesian coordinate system is simplest because it is merely necessary, for this purpose, to form, for instance, in a three-phase machine, from the voltages and currents of the three phases mutually shifted 120.degree., by means of a "3/2" coordinate converter, the corresponding Cartesian components fixed in space such "stator oriented" vector components are characterized here by the subscripts s1 and s2) of the corresponding stator current vector i.sub.s and the stator voltage vector u.sub.s. The vector e.sub.s of the EMF is then calculated, taking into consideration the stator resistance r.sup.s and the leakage inductance .tau. through addition, component by component, according to EQU e.sub.s =u.sub.s -r.sup.s .multidot.i.sub.s -.tau.-di.sub.s /dt.
The stator oriented Cartesian components of the flux vector .psi..sub.s are then obtained as the integral of the corresponding component of the EMF vector. In a coordinate system rotating with the flux vector, with the field-parallel coordinate axis .phi..sub.1 and the field-perpendicular coordinate axis .phi..sub.2, the EMF vector has the "field oriented" components e.sub..phi.1 and e.sub..phi.2, in the physical vector relation .psi.=.intg.edt, a rotational component related to the fluxfrequency .phi..sub.s (i.e., to the derivative of the angle .phi..sub.s between the axes .phi..sub.1 and s.sub.1) then appears in accordance with EQU .psi..sub.100 1 =.intg.(e.sub.100 1 +.phi..psi..sub..phi.2)dt EQU .psi..sub..phi.2 =.intg.(e.sub..phi.2 -.phi..psi..sub..phi.1)dt
The voltage model is therefore always operated as stator oriented
The open integrators required for EMF integration have a tendency to drift and must be stabilized, for instance, via a zero-point control oonnected into a feedback line. However, the correspondingly slow changes of the flux components are also suppressed at low operating frequencies along with the zero drift. In addition, an angle (phase) error is generated in stationary operation, which has an effect, likewise mainly at low frequencies, and leads to a disturbing misorientation if the reference values for the stator current are given as field oriented. However, these disadvantages are counterbalanced by the good dynamics of this voltage model.
However, it is also possible to determine a model value for the machine flux from the machine currents (i.e., the stator current vector i.sub.s and, in the case of a synchronous machine, also the field current i.sup.e) and the measured rotor position .lambda. or from the rotor speed .lambda., which is frequently advantageous from a measurement point of view. This "current model" simulates the processes occurring in the machine electronically, as far as they lead to the development of the flux. For this current model, the use of a field oriented coordinate system is of advantage, where the rotor time constant is taken into consideration as the time constant of a smoothing member and the current model forms a model flux frequency, from which the flux angle can be formed by integration
The conversion from one coordinate system into another coordinate system rotated by a given angle is accomplished by feeding the appropriate components of the vector to be transformed to a so-called "vector rotator", at the phase input of which a corresponding phase signal, for instance, sine and cosine of the angle of rotation, are applied.
In the current model, model parameters as accurate as possible much be set for the machine parameters, so that, for instance, temperature related changes of the rotor resistance lead to falsifications of the model flux in static as well as dynamic processes. For higher operating frequencies, the voltage model is therefore to be perferred, by at low frequencies, the current model leads to a better model value for the flux in spite of possible steady-state inaccuracies.
In the mentioned German Offenlegungsschrift No. 30 26 202.3, a combination of both models is therefore provided. According to the voltage model, there are formed from the machine currents and the machine voltages two components of a model EMF vector e.sub.s (u), from which the corresponding components of the flux vector .psi..sub.s (u) related to this voltage model are formed. The circuit operates here as stator oriented and contains for the formation of the flux one integrator for each Cartesian EMF component. For stabilizing these integrators, a component of this flux vector is impressed on a controller in a feedback line, the output signal of which is impressed as a correction quantity for correcting the corresponding component of the model EMF vector on the integrator input. To the reference input of these controllers is fed the corresponding component of a model flux vector formed by the stator currents and the rotor position .lambda. as the reference input .psi.*.
The controllers therefore receive at their inputs the Cartesian components, fixed in space, of the difference vector .psi..sub.s (u)-.psi.*.sub.s and furnish the Cartesian components, fixed in space, of a correction vector; by impressing it on the voltage model, the difference vector is levelled out on the average. Thereby, the voltage model, at least with respect to its stationary behavior, is slaved to the current model, so that the good dynamics of the voltage model is preserved, but the better stationary flux determination of the current model is utilized at low frequencies.
The outputs of the two known correction controls represent the stator oriented Cartesian components of a correction vector, which rotates essentially with the frequency of the vector .psi..sub.s. The controllers must therefore continuously process alternating quantities, which may be a disadvantage not only at high operating frequencies but, in particular, this presents difficulties if the method is to be implemented with a microprocessor.
It is therefore an object of the present invention to provide another way of determining the flux vector of a rotating field machine.